Indications and Procedures The overall goal of gene therapy is to correct an undesirable trait or disease by introducing a modified copy of a gene into a target cell. In most cases, the purpose is not to replace a defective gene in the host cell but rather to provide a new copy so that the correct protein can be expressed and the detrimental effects of the defective gene neutralized. While technically any genetic disorder may be treated by gene therapy, currently there are some limitations. First, the precise genetic mechanism of the disorder must be known, and it must be a single-gene defect. Second, scientists must know the complete genetic sequence of the gene, including regulatory regions, so that a functional copy can be delivered to the cell. Third, there needs to be an effective vector, or delivery system, for administering the correct copy to the target cells.
Generally, scientists classify forms of gene therapy as belonging to one of three types. Theoretically, the most effective form of this procedure is in situ gene therapy, which means that the genetic material is administered directly to the target cells. Unfortunately, it has been difficult to ensure that only target cells receive the genetic material, but there have been some successes. A second method injects the vector containing the genetic material into the fluids of the body. In this method, called in vivo gene therapy, the vector travels throughout the body until it reaches the target cells. A third mechanism, called ex vivo gene therapy, removes cells from the body to be exposed to the vector and then reintroduced back into the body. This method works especially well with undifferentiated stem cells.
Scientists have developed several mechanisms by which the genetic information can be introduced into the target cell. The most common is the viral vector. Viruses are used because typically they are very specific in the types of cells that they infect. Furthermore, their genomes are usually very small and well understood by scientists. The viruses that are chosen are derived almost exclusively from nonpathogenic strains or have been genetically engineered so that pathogenic portions of the genome have been removed. Common viral vectors are adenoviruses, retroviruses, and herpes simplex viruses. The choice of vector depends on the target and size of gene to be replaced. In each case, after the virus infects the target cell, the DNA is either incorporated directly into the host genome or becomes extrachromosomal.
Medical researchers are also investigating the use of nonviral vectors to deliver DNA into target cells. As is the case with viral vectors, these mechanisms must not disrupt the normal metabolic machinery of the target cell. One system, called plasmid DNA, utilizes small circular pieces of DNA called plasmids to deliver the genetic material. If small enough, the plasmids can pass through the cell membrane. Although they do not integrate into the host genome in the same way as viral vectors do, they are a simple mechanism and lack the potential problems associated with viral vectors. Another mechanism being studied is the packaging of the genetic material within a lipid-based vector called a liposome to ease transport across the membrane. In trials, however, both liposomes and plasmids have displayed a low efficiency in delivering genetic material into target cells.
Uses and Complications Since the early 1990s, numerous scientific studies have examined the potential effectiveness of gene therapy in treating diseases in mammalian model species, such as mice and monkeys. Using gene therapy, researchers have demonstrated that it may be possible to treat diseases such as Parkinson’s disease, sickle cell anemia, and some forms of cancer. Weekly scientific journals such as Gene Therapy report the status of these tests. Gene therapy trials in humans are a relatively recent development and represent the next stage in the treatment of human diseases. Severe combined immunodeficiency syndrome (SCID) was the first human disorder for which successful gene therapy was reported, and researchers have conducted and are conducting hundreds of clinical trials of gene therapy for other disorders, including Canavan disease, adenosine deaminase (ADA) deficiency, and cystic fibrosis. Medical researchers have suggested that, in the future, almost any genetic defect may be treatable using gene therapy. However, as of early 2015, the FDA had not approved the use of gene therapy as treatment for any disease; it is only available to the participants in these trials.
While gene therapy may appear to be the “silver bullet” for diseases such as cancer and Parkinson’s disease, the procedure is not without its risks. Since gene therapy using viral vectors was first proposed, scientists have recognized the inherent problems with the procedure. Since the technology does not yet exist to target the virus to insert its DNA directly into the specific gene of interest, the chances are that the viral vector will integrate the genetic information into the genome at some site other than the location of the defective gene. This means that the potential exists for the virus to insert itself into a regulatory or structural region of a gene and either render it unusable or impart a new function to the protein. Because of the size of the human genome (more than three billion bases) and the fact that less than 2 percent of the genome is believed to produce functional proteins, the odds of such an event occurring are relatively low. Given the large numbers of vectors used, however, this risk remains a real possibility.
Two cases illustrate the dangers associated with viral vectors. First was the death of a gene therapy trial volunteer at the University of Pennsylvania in 1999. The volunteer, Jesse Gelsinger, suffered from a form of liver disorder called ornithine transcarbamylase deficiency (OTC). OTC is identified as being the result of a single defective gene in a five-step metabolic pathway. Using an adenovirus, researchers sought to replace the defective gene causing OTC in Gelsinger. Shortly after the gene therapy was begun, Gelsinger developed a systemic immune response to the vector and died.
The second case is a story of both success and failure. A French research team at the Necker Hospital for Sick Children in Paris effectively used a retrovirus vector to treat a group of young boys with SCID. Also called “bubble boy disease,” SCID is a rare disorder in which the immune system is rendered inoperative. One form of the disease has been traced to a gene on the X chromosome. Using the procedure of ex vivo gene therapy, the researchers removed stem cells from the bone marrow of the boys and, using a retrovirus vector, delivered a functional copy of the defective gene into the cells. The cells were then reinserted back into the bone marrow. The procedure was successful in that all boys were cured of the disease. Thirty months later, however, one of the boys developed leukemia, which was followed four months later by a second case. Analysis of the boys’ DNA indicated that the inserted gene had disrupted a gene in which mutations had previously been shown to cause cancer.
While the number of individuals that have developed complications from gene therapy is relatively small, these cases do indicate the potential hazards of using a viral system and have accelerated the research into using nonviral systems such as liposomes and plasmids. Additional research is underway to develop a means of targeting a specific host gene for the insertion of the therapeutic DNA. Scientists are also investigating the possibility of developing a so-called suicide gene, or “off switch,” for the procedure that could terminate treatment if an error in insertion were detected.
Perspective and Prospects The process of gene therapy represents one of the more modern of advances in the life sciences. Since James Watson and Francis Crick proposed the structure of DNA in 1953, scientists have been suggesting the possibility of correcting genetic defects in a cell. It has been only since the early 1990s, however, that advances in biotechnology have enabled the actual procedure to be conducted.
The science of gene therapy actually began as enzyme replacement therapy. For patients suffering from diseases in which an enzyme in a metabolic pathway is defective, enzyme replacement therapy provides a temporary cure. In these cases, however, the therapy must be administrated continuously since the presence of a defective gene means that the body lacks the ability to manufacture new enzymes.
In the 1980s, enzyme replacement therapy was being used to treat a number of diseases including ADA deficiency, in which an enzyme in a biochemical pathway that converts toxins in the body to uric acid is defective. As a result, the toxins accumulate and eventually render the immune system ineffective. The modern era for gene therapy began in the early 1990s as scientists began to treat ADA deficiency with gene therapy. Through a series of trials, researchers learned that ex vivo treatment of stem cells proved to be the most effective mechanism for treating ADA deficiency with gene therapy. In 1993, researchers obtained stem cells from the umbilical cords of three babies who were born with ADA deficiency. After the correct genes were inserted into these stem cells, the altered cells were inserted back into the donor babies. After years of monitoring, it appears that the process has worked and the potentially fatal effects of ADA deficiency in these children have been reversed.
In the 2010s, promising results have also been found in using gene therapy to treat patients with thalassemia major, HIV, metachromatic leukodystrophy, Wiskott-Aldrich syndrome, choroideremia, and several kinds of cancer, but more clinical data is required before any of these treatments can be approved for regular use. A gene therapy treatment called Alipogene tiparvovec has also been demonstrated to successfully compensate for lipoprotein lipase deficiency, a cause of pancreatitis. In 2012, the European Commission authorized the commercial sale of this treatment under the name of Glybera, making it the first form of gene therapy to be approved for use outside of clinical trials.
Another promising area of gene therapy is the treatment of cancer. Cancer treatment using gene therapy would probably not involve replacing defective genes but rather “knocking out” those genes that are causing uncontrolled cell division within cancer cells. By arresting cell division, scientists can halt the spread of the cancer. This treatment would be especially useful in areas of the body where surgery is risky, such as brain tumors. The primary challenge at this stage is the targeting of the vector. A knockout vector would need to infect only cancer cells and not the other dividing cells of the human body.
A potential area of gene therapy that has yet to be exploited is germ-line gene therapy. Germ cells are those that are responsible for the formation of gametes, or egg and sperm cells. Since a germ cell contains only half the genetic information of an adult cell, it is relatively easy to replace genes using available procedures learned from biotechnology. Furthermore, since following fertilization the genetic material in the germ cells is responsible for the formation of all the remaining more than sixty-three trillion cells in the human body, any genetic change in the germ cells has the ability to be inherited by subsequent generations. Somatic cell therapy, such as that used to treat ADA deficiency and SCID, has the ability to influence only the affected individual, since these cells are not normally part of the reproductive process. Gene therapy in germ cells is currently considered unethical, but many consider it to be the mechanism of eliminating certain diseases from the human species.
While the use of gene therapy to correct human diseases may be stalled temporarily until technical obstacles are overcome, little doubt exists in the biomedical community that gene therapy represents the procedure of the future. At a fundamental level, gene therapy has the potential to be the ultimate cure for many ailments and diseases of humankind. For most of recorded history, medicine has been confined to the treatment of symptoms. Since the start of the twentieth century, advances have enabled enhanced surgical procedures, pharmaceutical drugs that alter or interact with the biochemistry of the cell, improved diagnostic techniques, and a deeper understanding of genetic inheritance. Gene therapy represents the ultimate preventive procedure.
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